Different protocols exist for establishing calls between endpoints coupled to a network. Two examples are the Session Initiation Protocol (SIP) and the H.323 protocols which are widely used as signaling protocols for Voice over IP (VoIP). A specification for SIP is available in RFC 3261 from the IETF and the H.323 standard is specified by the ITU.
Because SIP and H.323 are different protocols, an interworking function (IWF) must be used to connect calls between networks using the different protocols.